1. Field of the Invention
The present invention relates to nonwoven fibrous filter media which are particularly useful as prefilters on respirators.
2. Background Information
There is increasing interest by governmental agencies, the general public, and the work force in protecting individuals against the harmful effects of toxic materials such as lead fumes, silica dust, asbestos, and paint spray. A variety of face masks, respirators, and filters have been proposed to filter such toxic materials from the air.
U.S. Pat. No. 3,971,373 (Braun) discloses a particle-loaded small diameter fiber sheet product and respirators made therefrom. The sheet product comprises a uniform three-dimensional arrangement of discrete solid particles uniformly dispersed in a web of melt-blow small diameter fibers. The particles are physically held in the web, even though there is only point contact between the small diameter fibers and the particles, whereby the full surface of the particles is available for interaction with a medium to which the sheet product is exposed.
U.S. Pat. No. 4,215,682 (Kubik et al.) discloses a fibrous web electret which comprises melt-blown fibers prepared by extruding molten fiber-forming material through orifices into a high-velocity gaseous stream where the extruded material is drawn out into fiber and solidified in the fibrous form, the fibers having electrically charged particles and a persistent electric charge that is useful to enhance filtering properties of the web, the charge having a half-life of at least one week in a room-temperature, 100-percent-relative-humidity environment.
U.S. Pat. No. 4,294,599 (Grovesteen et al.) discloses an aerosol filter cartridge employing a prefilter having the form of a closely spiraled Fiberglas mat functioning to distribute captured aerosol particles substantially uniformly therethrough. The Fiberglas mat is placed edgewise against the final filtering components of the cartridge.
U.S. Pat. No. 4,334,901 (Ayes et al.) discloses a respirator cartridge which is designed to eliminate the traditional screen between the perforated cartridge bottom and its adjacent filter component, substitute Fiberglas for one of the usual two wool-felt components and minimize filter component-to-shell cementing operations along with reshaping of the initial aerosol contacting filter component for effecting greater than usual distribution of aerosol loading and lower inhalation resistance.
U.S. Pat. No. 4,600,002 (Maryyanek et al.) discloses a multiple ply, fold-flat, disposable respirator having, as one ply, a layer of a toxic gas or vapor absorbing material, and as another ply, an aerosol filter. The respirator has an inturned vertical seam covered with a strip of foamed elastomer.
U.S. Pat. No. 4,662,005 (Grier-Idris) discloses a pouch-like conformable face mask which includes a generally rectangular body portion formed from a filtration medium and having top and bottom edge portions and a pair of opposed side portions. An elastically extendible material is located at the periphery of the side and bottom edge portions of the generally rectangular body portion, mask to gather into a pouch or cup-like configuration.
U.S. Pat. No. Re. 32,171 discloses an electret fibrous web which is prepared by continuously feeding a film of highly molecular non-polar material; heating the film; stretching the film along the longitudinal axis thereof as defined by the path of movement of the film; homopolarly electrically charging at least one side of the film; fibrillating the charged film into fiber material; and collecting the fiber material.
Ogawa, in an article entitled "Development of Spunbonded Based on Thermoplastic Polyurethane," Nonwovens World, May-June, 1986. pp 79-81, describes a spunbonded nonwoven polyurethane elastic fabric developed by Kanebo Ltd. The fabric is made using a melt blown process which is different from a conventional melt blown process to produce fabric which is similar to that of spunbonded fabrics. The diameter of its filaments is not so fine as that of the usual melt blown fabrics, i.e., 0.5-2 microns, but apparently is closer to that of the spunbonded fabrics, i.e., 20-50 microns. The elasticity, dust catching capability, low linting, high friction coefficient, air permeability and welding characteristics of the urethane fabrics are discussed in the article.